[jdk21u] RFR: 8311249: Remove unused MemAllocator::obj_memory_range

[Aleksey Shipilev]

Clean backport to remove dead code, keep the codebases in sync (already backported to 17u), and make life easier for downstreams.

Additional testing:
 - [x] Local macos-aarch64-server-fastdebug build
 - [ ] GHA

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Commit messages:
 - Backport 711cddd899048236bbc3c0aa7e4ac0fc8aebe4d2

Changes: https://git.openjdk.org/jdk21u/pull/8/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=8&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311249
  Stats: 15 lines in 2 files changed: 0 ins; 15 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u/pull/8.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/8/head:pull/8

PR: https://git.openjdk.org/jdk21u/pull/8